Urban eco-tourism
Traveling within the city itself can hardly be called a real eco-tourism. Eco-adepts are still arguing, and I think they will speak on this topic for a very long time. But I am for the fact that the principles of sustainable travel can be implemented in all types of travel.
You will definitely not be able to enjoy the fresh air, silence and solitude in the city, but do not forget about the principles of eco-tourism on such trips. The environment should be protected: do not litter, use reusable dishes, be careful with the sights. If you want to make your vacation more conscious, walk more. Yes, in some cities it is difficult, but if you develop a route in advance and book hostels or hotels according to this route, you can minimize the use of transport.

Meet local customs. As strange as it may sound, many cities have so-called "urban legends" and customs. I really advise you to refrain from such traditions as "happy handshake", when, by touching some part of the statue, you can make a wish or just become happier. Remember about respect, such strokes very often harm monuments. But you can get a little bit of the local community, for example, by jogging in Central Park in New York or joining the general siesta in the cities of hot countries.
Choose eco-friendly hotels. Yes, there are such in the city. For example, in Portland, Maine, USA. The Mercury Inn is a small hotel in the heart of historic Portland. IT belongs to Tim Kar and Jacob Kruger, who try to follow all environmental standards. The hotel uses only energy-efficient lighting. The walls are painted with paints that comply with environmental standards. And in interior design, a lot of things from recycled materials are used. At the same time, the hotel itself is lovely and comfortable, located in a classic Victorian mansion.

You can also choose restaurants guided by the rules of eco-tourism. For example, when trying food in the markets. Very often, there are cafes with local cuisine in farmers' markets, even in large cities. You can ask the residents of the town what restaurants they would recommend. Locals can suggest exciting and unusual places.